1. Field of the Invention
The present invention relates to a conversion lens assembly for an automatic focusing system to be used in a lens exchangeable camera and more particularly, to a conversion lens assembly which is to be interposed between a camera body having a driving motor driven in response to a signal from a focus condition detection means and a driving force output member driven by the motor, and an exchangeable lens assembly having a driven member capable of being coupled with the driving force output member of the camera body to shift a focusing optical system within the exchangeable lens assembly for focusing.
2. Prior Art
Conventionally, there have been proposed various automatic focusing systems for use in lens exchangeable type cameras. In such known automatic focusing systems, there is included a system in which a focus condition detection means, a driving motor driven responding to a signal therefrom and a driving force output member to be driven by the motor are provided in a camera body, while in an exchangeable lens assembly to be attached to the camera body, a driven member coupled with the driving force output member of the camera body is provided so as to displace the focusing optical system for focusing. In the system as described above, arrangements conventionally proposed are limited to the case where exchangeable lens assemblies are directly mounted on the camera body, and no particular countermeasures have been proposed with respect to cases where, for example, telephoto conversion lenses which increase focal lengths of photographic lenses by 1.4, 2, 3 times, etc. or wide angle conversion lenses which reduce such focal lengths to 1/1.4, 1/2, 1/3, etc. are mounted behind the exchangeable lens assemblies.
Incidentally, with respect to the focus condition detection methods at the side of camera bodies, there have also been proposed various systems up to the present, which may be broadly classified, in terms of principles, into a phase difference system as disclosed in U.S. Pat. No. 4,333,007 and a contrast system as disclosed in U.S. Pat. No. 4,341,953.
The phase difference system as referred to above is a system by which an amount of the so-called out-of-focus condition can be detected together with its direction (i.e. both direction and amount of deviation of an image plane from a predetermined image forming surface can be detected). In order to effectively utilize such a system for an automatic focusing system of a lens exchangeable camera, it is necessary that the extent for driving the driving force output member in the camera body with respect to the detected amount of the out-of-focus can be determined at the side of the camera body. More specifically, the driving amount of the driving force output member is determined by such factors as a ratio of the amount of shift of the focusing optical system in the exchangeable lens assembly to the amount of the out-of-focus, a pitch on a lead of the focusing optical system shifting mechanism such as a helicoid thread, etc., and a reduction gear ratio possessed by the driving transmission system and the like, and is specific according to the kinds of the exchangeable lens assemblies. However, in such a lens as a zoom lens of a front group displacing focusing system, etc., the ratio of the focusing optical system shift amount to the amount of out-of-focus is varied by the zooming of the lens.
As a countermeasure for the above, in the lens exchangeable camera which employs various exchangeable lens assemblies there may be considered such an arrangement which, with the factors as described above being applied to the respective exchangeable lens assemblies as signals, compensations for the control of the driving amount of the driving force output member by the signals introduced into the camera body upon attaching of the exchangeable lens assembly to the camera body. It may also be considered as another countermeasure that, with respect to the driving amount of the driving force output member at the camera body side, no difference is provided for any exchangeable lens assembly to be attached, and at the side of the individual exchangeable lens assemblies, the pitch or lead of the focusing optical system shift mechanism and the reduction gear ratio, etc. are so modified as to be common to all of the exchangebale lens assemblies.
However, in the case where a conversion lens assembly (referred to as a rear converter hereinafter) is interposed between the exchangeable lens assembly and the camera body, the entire focal length as a photographing optical system is altered from the focal length of the single exchangeable lens assembly alone to a composite focal length of the exchangeable lens assembly and the conversion lens, with the out-of-focus amount corresponding to the focal length thus composed, while, on the contrary, since the focusing must still be effected only by the shift of the focusing optical system of the exchangeable lens assembly, there will be no correspondence between the image plane variation in the camera body and the shift amount of the focusing optical system in the exchangeable lens.
The situation as described above similarly applies to the case where any of the above countermeasures is taken, and when the correspondence is broken to a large extent there may arise a situation where focusing becomes difficult or impossible.
In order to solve the problem as described above, it is considered to impart to the camera body by some methods, the ratio of the amount of shift of the focusing optical system to the amount of out-of-focus, which is proper to the case where the rear converter is attached between the exchangeable lens assembly and the camera body. However, in the case where the driving motor in the camera body is a step motor or a motor subjected to pulse driving, the minimum shift amount of the focusing optical system is determined in correspondence to the unit driving amount of the motor, and when the minimum extension amount is not altered even if the rear converter is interposed between the exchangeable lens assembly and the camera body, the minimum variation amount of the image plane on the film surface is altered as compared with the case where only the exchangeable lens assembly is attached to the camera body. Particularly in the case of a telephoto rear converter which increases thc above composite focal length longer than the focal length only of the exchangeable lens assembly, since the ratio of the shift amount of the focusing optical system to the amount of out-of-focus becomes small, the minimum variation amount of the image plane grows larger, with a consequent deterioration in the focusing accuracy.
Meanwhile, there arises a similar problem also in the case where a focus detection system of the contrast type is adopted for an automatic focusing system of a lens exchangeable camera. More specifically, the contrast system is the system which is capable of detecting only the direction of the out-of-focus, and in the automatic focusing system of a lens exchangeable camera which adopts the above contrast system, when the focusing optical system of the lens is shifted by a step motor or a driving motor driven by pulses, problems as follows will be invited, if the minimum shift amount of the focusing optical system corresponding to the unit driving amount of the motor is not altered as compared with the case where only the exchangeable lens assembly is used, even when a rear converter is attached to the exchangeable lens assembly. In other words, in the actual system, there is present a time delay in a period from generation of a motor stopping signal following the focus detection to the actual stopping of the focusing optical system of the exchangeable lens assembly, and the image forming point undesirably overruns the predetermined image forming plane during that time. Particularly in the case of a telephoto converter, the amount of the overrun is enlarged, and therefore, it is possible that the focusing optical system undesirably passes through the focusing zone in spite of the generation of the motor stopping signal. In such a case, it becomes necessary to re-drive the driving motor in the opposite direction, thus resulting in an unpleasant feeling during use such as an unstable image in a viewfinder. Meanwhile, even on the assumption that the time delay from the generation of the motor stopping signal to the actual stopping of the focusing optical system is zero, since the amount of variation of the image forming position by the increment driving of the driving motor is also altered by the use of a rear converter, the focusing accuracy is lowered when a telephoto converter is used resulting in such an inconvenience that the focusing optical system does not stop within the focusing zone in many situations.
It is to be noted here that in the case where the focus detection of the phase difference system as described above is effected, if the countermeasures as explained are taken, the driving amount of the camera body side driving force output member can be preliminarily determined at the side of the camera body through focusing detection of one time (in the case where the relation between the shift amount of the focusing optical system and the out-of-focus amount is in an approximately linear relationship) and/or several times (in the case where the relation between the shift amount of the focusing optical system and the out-of-focus amount is non-linear), and therefore, there is an advantage in that an accurate focusing may be effected at high speeds. However, if the problems as stated previously should take place by the use of the rear converter, such advantage may be undesirably lost completely, and therefore, the problems as described so far are very serious especially for the automatic focusing system of a lens exchangeable camera in effecting the focusing detection of this system.